1. Field of the Invention
The present invention relates to a semiconductor pressure transducer apparatus comprising strain gauge elements which are formed integrally in a diaphragm of a single crystal of such as silicon (Si) by injecting an impurity into the diaphragm at one surface thereof so as to exhibit a piezoresistance effect.
2. Desciption of the Prior Art
As is known in the art, when a circular diaphragm surface is formed by slicing or cutting a semiconductor single crystal which is made to exhibit a piezoresistance effect through the injection of an impurity, there exists at least one crystallographic axis along which the maximum piezoresistance effect is produced, although the magnitude thereof may vary depending on the crystal plane forming the diaphragm surface.
It is also known that, in the case of strain gauge formed of a semiconductor material injected with a P-type impurity, the static characteristic of change in the piezoresistance as a function of strain or stress is such that the piezoresistance is increased with increasing tensile stress such that the rate of resistance increasing is gradually increased, while for the compressive stress the piezoresistance is decreased with increasing compressive stress such that the rate of resistance decreasing is gradually decreased, that is, following a saturation characteristic.
Generally, the non-linearity of the strain-resistance characteristic of a strain gauge element is classified into two different types, that is, a negative non-linear characteristic in which the rate of resistance increasing becomes larger as the strain increases and a positive non-linear characteristic in which the rate of resistance increasing becomes smaller as the strain increases.
Back to the prior art, by taking advantage of the above-mentioned phenomenon, the linearity of the static resistance change characteristic of the strain gauge device has hitherto been improved by combining a pair of strain gauge elements whose resistances are changed oppositely to each other in a so-called bridge circuit so that the non-linearities in change of the piezoresistances of the individual gauge elements may be compensated by each other.
Furthermore, it has also been proposed that the pressure-responsive section of a strain gauge diaphragm is made by a semiconductor crystal to have a surface constituting a crystal plane such as (100) of the semiconductor crystal, and a first strain gauge element is formed on the diaphragm surface by injecting an impurity in a linear region extending on the surface in parallel with a crystallographic axis (110) along which a maximum piezoresistance effect is produced, while a second strain gauge element is formed by injecting an impurity in a linear region extending in the direction normal to the crystallographic axis (110) along which a maximum piezoresistance effect is also produced; herein the first and second strain gauge elements are located with an equal radial distance from the center of the circular diaphragm thereby to obtain the same resistance change for both the gauge elements. For example, reference is made to an article entitled "Solid State Digital Pressure Transducers" of IEEE Vol. ED-6, No. 10, pp. 867 et seq. Oct. 1969.
In connection with the hitherto known strain gauge devices such as described above, it should be noted that the prior art strain gauge device has not improved the non-linearity in stress-resistance characteristic of each strain gauge element at all, but has allowed a great non-linearity of each of the gauge elements. With such an arrangement, however, restriction will be inevitably imposed on the degree of attainable compensation. If there is even a little difference between the individual gauge elements in respect of the piezoresistance effects thereof, unsatisfactory compensation will result, involving errors in the measurements. For this reason, the strain gauge diaphragm of the known structure can not be reliably employed in a transducer for measurement of pressure or differential pressure, if a high accuracy is required.